Wireless telephony, e.g. cellular phone use, is a widely-used mode of communication today. Variable rate communication systems, such as Code Division Multiple Access (CDMA) spread spectrum systems, are among the most commonly deployed wireless technologies. Industry Standard (IS-95) provides details on conventional CDMA standards. Because of increasing demand and limited resources for this communication medium, a need arises to improve the capacity, fidelity, and performance of devices and methods for wireless communication.
Referring to prior art FIG. 1A, an illustration of multipath signal propagation between a conventional base station and a cell phone is shown. A conventional base station 104 transmits a signal to a mobile unit 102, e.g., a cell phone. Typically, the signal contains pilot information, that identifies the base station, and data information, such as voice content. A signal that can be transmitted directly to mobile unit 102 without interference, such as first signal 106a, which provides the strongest signal. However, given the power limitations at which base station 104 can transmit the signal, and given the noise that a signal may pick up, a need arises to improve the power and the SNR of the signal captured at the mobile unit.
Conventional methods combine transmitted signals that travel different paths to mobile unit 102. The multiple paths arise because of natural and man-made obstructions, such as building 108, hill 110, and surface 112, that deflect the original signal. Because of the paths over which these other signals travel, a time delay and performance deterioration intrinsically arises in the synchronization-sensitive and noise-sensitive data that is transmitted from base station 104 to mobile unit 102. To provide the strongest possible signal to a mobile unit, two or more of the signals from these multiple paths, e.g. path 106a-106d, may be combined.
Corruption of a transmitted signal falls into two general categories: slowly-varying channel impairment and fast fading variation. Slowly-varying channel impairment arises from factors such as log-normal fading, or shadowing caused by movement or blocking from objects, as shown in prior art FIG. 1A, or from slow fading. Slower variations, e.g., sub Hz, determine in effect, the “availability” of the channel. In contrast, only the fast fading variation affects the details of the received waveform structure and the interrelationships of errors within a message. Interference on a signal can be caused by moving objects that temporarily block the signal, such as moving object 113 that interferes with signal 106b of prior art FIG. 1A. Based upon the characteristic differences of these signals, a need arises for a method of capturing a signal while avoiding the detrimental characteristics of fast fading or short fading variation encountered at the receiving unit.
Referring now to prior art FIG. 1B, a graph of the signal strength of two conventional multipath signals over time is shown. These curves are provided to illustrate how a conventional demodulation finger would react to fading signals. Graphs 100b and 101b illustrate some weaknesses of the conventional method of managing assigned fingers. These weaknesses will be more specifically described in a following figure, prior art FIG. 1C. Graphs 100b and 101b have an abscissa 122 of time and an ordinate 120 of signal strength, e.g., signal-to-noise ratio (SNR). SNR can be a received pilot energy per chip, Ec, divided by a total received spectral density (noise and signal), Io, thus yielding an Ec/Io ratio.
Third multipath signal 106c in graph 100b and second multipath signal 106b in graph 101b are shown as exemplary multipath signals received at mobile unit 102. Third multipath signal 106c exceeds threshold 126 early in time, e.g., as shown where solid line changes to dashed line. At time 122a, third multipath signal 106c fails to meet threshold 126. Shortly after time 122a, third multipath signal 106c regains its signal-strength value and exceeds threshold 126.
In contrast, second multipath signal 106b, shown in graph 101b, only satisfies threshold 126 after time 122b. Even then, second multipath signal 106b falls below threshold 126 shortly thereafter, at time 122c. Both signals 106b and 106c show fast fading variation, which is caused by interfering object 113 in the case of signal 106b, as shown in prior art FIG. 1A. Second multipath signal 106b would be deassigned 123 when it's signal-strength fell below threshold 126, then reassigned 124 when it rose back above threshold 126. This condition of continuously assigning, deassigning, and reassigning, at a high frequency is known as thrashing.
Referring now to FIG. 1C, a flowchart of a conventional process used for implementing fingers in a communication device is shown. Flowchart 100c begins with step 1002. In step 1002, an inquiry determines whether an assigned signal fails to meet a threshold for combining. If an assigned signal does fail to the single threshold, then flowchart 100c ends. If the assigned signal satisfies the threshold, then flowchart 100c ends. In step 1004, the finger assignment is immediately deassigned, e.g. because it failed to meet the threshold. Following step 1004, flowchart proceeds to step 1006. In step 1006, the communication device waits for the searcher to assign a new finger.
Prior art FIG. 1C presents several problems associated with the conventional management of assigned fingers. The first problem deals with thrashing. The second problem deals with unnecessary latency. In step 1002, the only criteria by which fingers are deassigned is a single threshold for combining the signal. This single threshold is shown in prior art FIG. 1B as threshold 126. By using only a single threshold, third multipath signal 106c is immediately deassigned, per step 1004, as soon as it fails threshold 126, e.g., at time 122a. Because of this limitation, one of the demodulating fingers must now wait for the searcher to identify a new multipath signal to be assigned, e.g., per step 1006. This latency is shown as the delay 128 between time 122a and 122b, where third pilot 106c is deassigned and second multipath signal 106b is assigned.
This latency, caused by reassignment, appears to be unnecessary in the case presented in prior art FIG. 1B. This is because third multipath 106c returns back to a satisfactory SNR level shortly after deassignment at time 122a, e.g., which is typical performance for short fade performance. In contrast, second multipath signal 106b, substituted for third multipath signal 106c, appears to be an inferior candidate because it fails the threshold more frequently over time. The latency may have an adverse effect on the quality of the signal presented by mobile unit 102 to a user, especially if it occurs frequently or unnecessarily. Hence, a need arises to prevent the problem of latency caused by frequent or unnecessary changes in finger assignment.
In a different scenario, if no other multipath signals are available for demodulation, and a demodulating finger is available, then second multipath signal 106b may be constantly assigned and deassigned from the given demodulating finger based on its performance. That is, second multipath signal 106b frequently crosses the threshold value, thereby causing the communication device to frequently assign, deassign, and reassign a multipath signal to a demodulating finger that has no other worthy candidate multipath signals. This phenomenon of frequent assigning and deassigning is referred to as “thrashing.” Unfortunately, thrashing consumes a significant amount of system resources, such as CPU operations, by constantly performing tasks such as assigning and deassigning. Furthermore, thrashing may downgrade the quality of the output signal from the mobile unit 102. This is because the frequent changes in finger assignment, and its associated latency effects, may cause a perceptible degradation in the composite signal provided by the communication device to a user. Consequently, a need arises for a method of managing assigned fingers that avoids the problem of thrashing, and its associated side-effects.
In summary, an apparatus and a method are needed to improve the capacity, fidelity, and performance of digital communication. In particular, a need arises to improve the power and the SNR of the signal captured at the mobile unit. That is, a need arises for a method of capturing a signal while avoiding the detrimental characteristics of fast fading variation encountered at the receiving unit. Specifically, a need arises to prevent the problem of latency caused by frequent or unnecessary changes in finger assignment. Finally, a need arises for a method of managing assigned fingers that avoids the problem of thrashing.